Elevator control and safety requirements dictate that the speed and position of an elevator car must be monitored accurately. Knowing the speed of an elevator car is important for control purposes. For example, calculating the elevator car travel time or the travel distance necessary for proper deceleration or acceleration both require an accurate rate of speed value. Knowing the position of the elevator car is also important for control and safety purposes. Accurate positional data for the elevator car, for instance, is essential to ensure that the car aligns with each landing correctly. Moreover, knowing the position allows the control system to begin deceleration at the proper position, thereby allowing the car to stop smoothly at each landing.
Prior art teaches that elevator car speed may be monitored through a conventional governor rope assembly mainly comprising a governor rope, a governor sheave, a tension sheave, and mechanical linkages which operate safety brakes. The governor sheave typically employs a centrifugal device which engages a brake, producing a drag on the governor cable in an overspeed condition. If the overspeed condition persists, the a second independent safety brake system mechanically deploys and the elevator car is safely brought to a halt.
A disadvantage of this type speed control system is the frictional wear which naturally occurs in a governor rope assembly. The greatest problem with this type wear is that it is often visually undetectable. In addition, frictional wear also occurs during an overspeed braking condition. Eventually the frictional forces will wear the governor rope enough to necessitate replacement thereof.
A further disadvantage of a governor rope assembly is the required maintenance. The governor rope, the sheaves, and the linkages all must be periodically cleaned, lubricated, or replaced. All maintenance requirements are considered a burden to those skilled in the art, and therefore represent an undesirable feature in an elevator system.
Prior art discloses an elevator positioning system independent of the elevator speed control. Conventional positioning systems typically include a sheave located in the machine room of the elevator and a perforated tape running up from the elevator car, wrapping around the sheave, and back down to the counterweight. The perforated tape travels up and down with the elevator, rotating about the sheave. A disadvantage of this type positioning system is the fatigue created in the tape by the constant wrapping of the tape around the circumference of the sheave.
A disadvantage of the aforementioned prior art is that the speed and monitoring systems are independent from one another. Two independent systems create a redundancy in some of the required hardware as well as a greater demand of hoistway space.